1. Field of the Invention
The present invention relates to an information recording apparatus for optically recording multi-leveled information with high density in a phase-change recording medium comprising a recording layer which comprises a phase-change recording material.
The present invention also relates to an information reproducing apparatus for optically reproducing multi-leveled information recorded in the phase-change recording medium.
The present invention also relates to an information recording and reproducing apparatus for conducting the above-mentioned recording and reproduction.
The present invention also relates to an information recording method and an information reproducing method which can be respectively used in the information recording apparatus and in the information reproducing apparatus.
The present invention also relates to a phase-change recording medium for use in the information recording apparatus, the information reproducing apparatus, the information recording and reproducing apparatus, the information recording method, and the information reproducing method.
2. Discussion of Background
As one of optical recording media which are capable of recording information therein and reproducing recorded information therefrom with the application of a laser beam thereto, there is conventionally known a rewritable recording medium comprising a recording layer which comprises a phase-change recording material. The phase-change recording material used in the recording layer of the rewritable recording medium is capable of performing reversible phase changes between an amorphous phase and a crystalline phase, so that the rewritable recording medium has such excellent advantages over conventional recording media that (1) recording and erasing of information can be carried out, using a simple optical system and that (2) new information can be easily recorded in the recording layer, while previously recorded information is being erased therefrom at the same time.
Generally, in such a phase-change recording medium as mentioned above, an amorphous portion in the recording layer is used as a recorded portion, while a crystalline portion in the recording layer is used as an erased or non-recorded portion,
The recording of information is actually carried out by applying a laser beam having a power on a recording power level to a recording layer in a crystalline state of the recording medium to heat the laser-beam applied portions of the recording layer to the melting point thereof or to a temperature above the melting point thereof, and then rapidly cooling the heated portions to form recording marks in an amorphous state in the recording layer so as to correspond to the information to be recorded.
The recorded information can be erased by the steps of (1) applying a laser beam having a power on an erasing power level to the recording layer of the recording medium to elevate the temperature of the recording layer to a crystallization temperature thereof, and (2) then gradually cooling the recording layer, thereby crystallizing the recording marks in the amorphous state in the recording layer.
The recording marks are in the amorphous state and the background thereof is in the crystalline state. The recording marks in the amorphous state and the background in the crystalline state have a different reflectivity or a different phase in the light reflected therefrom. By use of such differences between the recording marks and the background thereof in the recording medium, the changes in the quantity of the light reflected by the recording medium can be detected, and recorded information can be reproduced by detecting the changes in the quantity of the light reflected by the recording medium. The recording medium may be in the form of a disk.
In the above-mentioned information recording method using the phase changes in the recording layer, in order to attain high recording density, it is necessary to reduce the size of the recording marks. However, there is a limit to the reduction in the size of the recording mark.
Furthermore, in the phase-change recording by forming recording marks in the above-mentioned heat mode, the shape of the recording marks is act to be significantly affected by the variation in the amount of light of the laser bean. When the shape of the recording marks is varied, as a matter of course, the recording of information and the reproducing of recorded information cannot be carried out precisely in a stable manner.